I just finished AS edexcel economics this year and I found it to be the easiest of my subjects (maths, fm, phys)
Its one of them subjects which requires low-medium effort to maintain an A compared to something like physics. As long as you understand/memorise all the content, you should be perfectly fine.
One of the best methods that I found for AS was to basically just read a paragraph (of solid content) and repeat it without looking at it. This worked well as most of the exam is just regurgitating everything you've learnt from the textbook
As for Cambridge, its is strongly recommended to take further maths as a fourth AS. Not only because most candidates will have it but because it will help greatly as I heard the course there is extremely mathematical. I self-taught fm this year (and will do next year) and it is not much of a step up from normal maths as long as you are willing to put the work in.